The present invention relates to fluid condition monitoring utilizing a sensor providing an electrical signal indicating in real time the chemical condition of the fluid to be monitored. Sensors of this type are known to employ impedance spectroscopy techniques; and, an example of such a sensor is that shown and described in U.S. Pat. No. 6,278,281 Bauer, et al granted Aug. 21, 2001 in which a pair of spaced electrodes are sequentially excited at a relatively low frequency for determining the electrochemical interaction at the electrode surface and at a relatively high frequency for determining the bulk impedance of the fluid. The current is measured at both excitation frequencies and the impedance computed for each current measurement and the impedance differential computed which enables determining the fluid condition by a comparison of the computed impedance differential with that determined for known conditions of the fluid as determined by chemical analysis.
It has been proposed to use such devices for monitoring fluid condition in power transmissions and for real time monitoring of lubricating oil in combustion engines. A further example of such a sensor application for engines is that shown and described in U.S. Pat. No. 6,377,052, McGinnis, et al granted Apr. 23, 2002 in which the spaced electrodes are spirally wound on a dipstick for insertion into the engine crankcase.
Devices of the aforesaid type employing impedance spectroscopy may utilize the electrode arrangements of the type employing interdigitated planar arrays of electrodes or the aforementioned spiral arrangement or concentric radially spaced tubular electrodes such as for example those taught in U.S. Pat. No. 6,433,560 issued to Hansen et al. and granted Aug. 13, 2002.
The aforesaid Bauer, et al. patent describes in FIG. 15 thereof the impedance determined at the aforesaid high and low frequencies for automatic transmission fluid in the new condition and after a limited number of vehicle miles in service.
However, since actual vehicle service conditions depend upon the type of vehicle operation and the loading and environment during such operation, it has long been desired to provide a sensor which can provide over the service life of the vehicle a real time indication of the fluid condition based upon the actual chemical characteristics of the fluid and to indicate the amount or percentage of estimated remaining useful life (RUL) based upon the current condition of the fluid.